This invention relates generally to radio frequency antenna and more particularly to radio frequency antenna adapted to operate with radio frequency energy having any one of a variety of polarizations.
As is known in the art, it is frequently desirable to use an antenna element which may operate with any one of a variety of polarization (i.e. linear or circular). One type of antenna element capable of such operation is sometimes referred to as a "double ridged" horn. One antenna element of such type generally would include a vertical feed and an independent horizontal feed, the phase centers of such feeds being coincident. For circular polarization the two feeds are fed with radio frequency energy having a quadrature phase difference. In order to provide efficient matching to free space over a relatively wide frequency band, say in the order of 3.5 to 1, it is generally required that the width of the horn be larger than half the wavelength at the nominal operating frequency of the antenna and sometimes be as large as one wavelength. In an array antenna, a plurality of antenna elements are provided in order to attain a relatively wide scan angle, say in the order of 120 degrees. In such array, it is generally required that the phase centers of adjacent ones of the plurality of antenna elements be displaced by less than one half wavelength. It follows then that while a double ridged horn antenna may be adapted to operate with radio frequency energy having circular polarization, such an antenna element may not be readily used, because of its size, in an array antenna having relatively wide scan angles.
In another type of array antenna adapted to provide a variety of polarization each one of the antenna elements includes an orthogonally disposed pair of printed circuit notch shaped antenna elements. One such type of antenna is described in U.S. Pat. No. 3,836,976 entitled "Closely Spaced Orthogonal Dipole Array," inventors George J. Monser, George S. Hardie, John R. Ehrhardt and Terry M. Smith, issued Sept. 17, 1974, and assigned to the same assignee as the present invention. While such antenna is adapted to operate with circularly polarized radio frequency energy over relatively wide scan angles and over a relatively wide band of frequencies, such antenna is limited in its power handling capability and hence is not suitable for use in those applications where such antenna is fed by a transmitter adapted to transmit relatively large amounts of power.